The transmission of optical energy through ocean water is subject to a number of limitations due to absorption and other loss mechanisms. Although a few applications exist for use of optical imaging systems, the primary means for probing the ocean is by acoustics, since the water is essentially transparent to sound. While light can travel on the order of tens of meters in the ocean, sound can travel thousands of kilometers.
The two most widely used acoustic techniques for probing the ocean are "active", or "echo-ranging", where an energetic acoustic signal is transmitted into the ocean, with the presence of the objects being inferred from the echoes they produce, and "passive", or "direct", in which the receiver detects sounds produced by the object to be imaged. While both of these techniques are effective, they are limited by, among other things, background noise. Also, the signal produced, which is usually a "blip" on a screen requires interpretation by skilled personnel. Detailed images are not available.
The ocean below the surface is a noisy environment. This ambient noise is generated by a number of sources including breaking waves, wind, spray, rain, man-made sources such as ships, underwater drilling and exploration, and marine mammals. This ambient noise is incoherent, radiating randomly in all directions from the source.
The ambient noise within the ocean is analogous to daylight in the atmosphere. The incoherent light which comprises daylight allows objects to be seen without relying on active or passive methods of detection. The ability to see an object in daylight may be enhanced by using lenses, e.g., cameras, microscopes and binoculars, to focus the light at the viewer's eye or on film or an image detector. Following the analogy, it should be possible to use ambient noise in the ocean as "acoustic daylight" for seeing in the ocean.